Optical circulators are commonly used in optical communication systems and optical measurement systems. FIG. 1 shows a three-port circulator 10 that has ports 1, 2, and 3. Each of ports 1, 2, and 3 can be coupled to a Polarization Maintenance (“PM”) fiber. As shown in FIG. 1, a polarized optical signal S1 entering port 1 exits from port 2 as a polarized optical signal S2. A polarized optical signal S2′ entering port 2 exits from port 3 as a polarized optical signal S3.